1. Field of the Invention
The present invention relates to a speed control apparatus for a movable equipment. More specifically, the present invention relates to a control apparatus which should not change the speed during operation, such as, for example, a control apparatus which is suitable for application to a motor for a VTR and a digital control apparatus which is suitable for the type of control apparatus.
2. Description of the Related Art
A drive motor for a VTR (video tape recorder) is required to have a constant speed. Variation in the speed (nonuniformity in revolution, speed ripple, torque ripple) disturbs the picture of a VTR, thereby greatly impairing the reliability and the quality of the VTR.
A conventional apparatus in this field mainly uses a DC motor. A brushless motor, which allows the speed to be varied freely and simply, has recently been increasingly adopted for such an apparatus.
Since a brushless motor is not provided with mechanical brushes, various problems caused by abrasion of brushes and commutators or abrasion powder of the brushes and commutators are eliminated. On the other hand since the number of the interlinking magnetic fluxes at conducting coils of a 120-degree conducting type brushless motor is different depending upon the position of the rotor, a torque ripple is caused, thereby producing nonuniform rotation (speed variation) during operation.
If it is assumed that the number of interlinking magnetic fluxes of the coil of a driving phase is K(.theta.) (.theta. represents a position of the movable equipment), the torque generated is represented by K(.theta.)I (I represents a current passing through the coils). When a current is constant, a torque ripple is generated in proportional to k(.theta.) like the number of interlinking magnetic fluxes.
An example of making the current 1/K(.theta.) in correspondence with the rotational position of the rotor is disclosed in Japanese Patent Laid-Open Publication 79694/1980.
This is a method of obtaining a constant torque by storing the information on 1/K(.theta.) corresponding to the rotational position of the rotor in a ROM (Read Only Memory) in advance and correcting the current instruction 1/K(.theta.) corresponding to the rotational position of the rotor.
In the invention disclosed in Japanese Patent Laid-Open Publication 79694/1980, however, since the number K(.theta.) of the interlinking magnetic fluxes varies due to the nonuniformity in the magnetized state of a permanent magnet and the arrangement and the wound state of coils, it is difficult to accurately detect the compensation signal 1/K(.theta.) in each motor.
In a brushless motor having a core, the cogging torque also generates a large revolution variation. Since the magnitude and the phase of the cogging torque greatly vary due to nonuniform magnetized state and assembling error, it is impossible to perfectly compensate the torque ripple and speed variation on the basis of the fixed data stored in the ROM.
In the case in which the cogging torque and the torque ripple are unknown, it is possible to suppress the torque ripple by a learning control method in which independent integration terms used exclusively for speed control are prepared independently in correspondence with the rotational positions and these integration terms are changed over sequentially in correspondence with the position of the rotor.
However, this method requires many RAMs (Random Access Memories) in correspondence to the number of pulses generated from an FG (Frequency Generator), thereby disadvantageously making a control apparatus complicated and expensive.
And, as a system for eliminating nonuniformity in revolution, the present inventors proposed a system for reducing the nonuniformity in revolution by detecting the harmonic wave component in nonuniform revolution and correcting a current instruction (torque instruction) by this harmonic wave component, and filed this system as Japanese Patent Laid-Open Publication 218380/1989. The main structure of the system is composed of an object of control (motor) for producing a controlled variable (number of revolutions), a detecting portion (encoder, counter) for detecting the controlled variable (number of revolutions), a discriminating portion for calculating the deviation from the target value and the measured value output from the detecting portion, and a control unit for determining the manipulated variable output to the object of control (motor) from the deviation. The control unit and the discriminating portion are provided with digital operators such as a microcomputer. This apparatus is a digital control apparatus for outputting a manipulated variable at every predetermined sampling period on the principle of detecting a variation component at every predetermined sampling period which is contained in the manipulated variable (number of revolutions) and the deviation and reducing the varying harmonic wave component contained in the object of control by correcting the manipulated variable by a correction value having the same period as the variable harmonic wave component.
In the above-described prior art, since the fluctuation of a controlled variable contains a plurality of harmonic wave components, it is necessary to provide a plurality of harmonic wave component detecting and correcting means. In this case, since two harmonic wave components are detected and corrected in one sampling time, compensation takes a long time, and since the sampling for compensation is the same in a long-period harmonic wave component detecting and correcting portion and a short-period harmonic wave component detecting and correcting portion, it is impossible to shorten the response time of the short-period harmonic wave component detecting and correcting portion.